Method for producing adherent,ductile zinc coating on ferrous substrates by vacuum deposition

ABSTRACT

A ZN COATING OF UP TO SEVEN MILS THICKNESS IS DEPOSITED ON A CONVENTIONALLY CLEANED, FERROUS SUBSTRATE BY CONDENSATION OF ZN VAPORS IN VACUO. TO MAKE THE COATING ADHERENT, IT IS PASSED OUT OF THE VACUUM CHAMBER AND RAPIDLY HEATED TO A TEMPERATURE OF 750*F.-1000*F. AND THEN IMMEDIATELY QUENCHED. THE ELASPED TIME FROM BEGINNING OF HEATING TO THE CONCLUSION OF THE QUENCHING MUST BE NO GREATER THAN ABOUT 25 SECONDS.

United States Patent 'Oflice 3,788,877 Patented Jan. 29, 1974 3,788,877METHOD FOR PRODUCING ADHERENT, DUCTILE ZINC COATING ON FERROUSSUBSTRATES BY VACUUM DEPOSITION Lawrence E. Helwig, Hampton Township,Allegheny County, and Michael V. Murray, Monroeville, Pa., assignors toUnited States Steel Corporation No Drawing. Filed July 20, 1972, Ser.No. 273,573 Int. Cl. C23c 13/02 US. Cl. '11762 5 Claims ABSTRACT OF THEDISCLOSURE A Zn coating of up to seven mils thickness is deposited on aconventionally cleaned, ferrous substrate by condensation of Zn vaporsin vacuo. To make the coating adherent, it is passed out of the vacuumchamber and rapidly heated to a temperature of 750 F.1000 F. and thenimmediately quenched. The elapsed time from beginning of heating to theconclusion of the quenching must be no greater than about 25 seconds.

The invention is directed to a method for the vapor deposition of Zncoatings in vacuo and is specifically directed to a method forsignificantly improving the adherence of such vapor deposited coatingsto ferrous substrates.

Zn coatings have been applied to ferrous strands (sheet, strip, wire,rod, etc.) by hot-dipping electroplating, cementation, hot spraying andby vacuum deposition. Each of these techniques is accompanied by certainadvantages and disadvantages. Cementation is a slow process, not welladapted for high speed continuous application and produces an iron-zincalloy coating which lacks ductility. Wire spraying does not yielduniform coatings. Electroplating is adapted to continuous application ofuniform, ductile coatings. However, the process is very slow andtherefore is only utilized, commercially, for very thin films.Therefore, only hot-dipping and vacuum deposition are commerciallypracticable for the continuous, high speed application of thick zinccoatings. For hot dip coatings to be adherent, the molten zinc bath mustcontain a small amount of aluminum, which has the disadvantage of makingthe coating susceptible to staining or white rusting. Additionally, if asmooth surface is required, the hot-dipped spangled surface can only beminimized by subsequent treatments such as temper-rolling, annealing orsteam treatment. Even when such subsequent treatments are employed,coating smoothness and ductility are almost impossible to achieve forcoatings thicker than about two mils, partly because of the brittleiron-zinc alloy layer that forms at the zinc-iron interface.

Thick, smooth, pure Zn coatings, without a brittle alloy layer have beenobtained by vacuum deposition techniques. However, a number of ratherexpensive expedients have been required to make the coating adherent;since, without such expedients, the Zn so deposited can be readilyremoved with transparent, adhesive tape. In one such process (US. Pat.3,326,177) for providing an adherent coating, the ferrous substrate mustbe extensively cleaned, i.e. by glow-discharge in hydrogen, immediatelybefore condensation of the Zn vapors. In another (US. Pat. 3,278,331), avery thin Zn coating is deposited, which is then heated in vacuum toalloy with the substrate. Subsequently, the substrate is cooled invacuum and only then coated with the remainder of the Zn to achieve thedesired thickness. In yet another process, a bonding layer, e.g. iron,is deposited at 350 F.; a thin Zn layer is then deposited at about thesame temperature, and the strip is then cooled prior to deposition ofthe remainder of Zn. Even when such expensive expedients are employed,it is often the case that coatings thicker than about one mil crack andlose adherence in a simple handkerchief bend test.

It has now been found, that thick, adherent Zn coatings may be depositedon ferrous substrates which have been cleaned in conventional manner,i.e., merely to remove surface grease and oils. The Zn is deposited invacuo at a pressure below about 4 10- mm. Hg. No. heating of thesubstrate is required and it is preferable that the substratetemperature be below 200 F. At this point in the process, the depositedcoating is not adherent, and care must be taken to prevent abrasion bythe exit seals. If the substrate surface is roughened prior todeposition, then some slight abrasion can be tolerated. After coating iseffected, the substrate is removed from the vacuum chamber and israpidly heated to a temperature of 750 to 1000 F. (depending on theatmosphere employed) and immediately quenched to below 200 F. The rapidheating and quenching is critical, so that the whole process occurs in aperiod of no more than about 25 seconds. No special atmosphere isrequired for the heating step. The only requisite being that thepressure is sufficient to prevent the re-evaporation of the Zn. Thus,for example, air, neutral or reducing gases, at atmospheric pressure maybe utilized.

Accordingly, it is an object of this invention to provide an economicalmethod for the vacuum deposition of Zn coatings.

It is a further object of this invention to provide a thick vacuumdeposited Zn coating exhibiting a superior combination of ductility andadherence.

It is another object of this invention to provide a method for obtainingthick, spangle free coatings.

These and other objects and advantages of this invention will becomemore apparent in the course of reading the following detaileddescription, taken in conjunction With the appended claims. Thefollowing specific exam ples are offered to illustrate the importance ofthe subsequent heat treatment of this invention and the parametersrelevant thereto.

Sheet and wire speciments were coated by evaporating reagent grade Znshot from a radiant-heated steel tray. All the sheet specimens and someof the Wire specimens were solvent cleaned with toluene and alcoholprior to coating. Those wire specimens, which were not solvent cleaned,were alkaline cleaned and then pickled in citric acid prior to coating.Specimens were at room temperature (except for those indicatedotherwise) at the start of Zn condensation and were only heated by thelatent heat given up by the Zn vapors as they condensed. Coatings ofvarying thickness were achieved at a variety of deposition rates, inperiods of from about 0.5 to 3 minutes. Sheet specimens were coated atpressures of 1 10- mm. Hg; wire specimens at pressures of from 1 l0- to2 10- mm. Hg. The coated specimens were removed from the bell-jar vacuumcoating chamber and post-heated (in the cases indicated) by resistancemethods to various temperatures and then quenched in water. Thetemperature of each speciment during heating and cooling was monitoredby a thermocouple attached thereto. Adherence was measured by a180-degree fold of the TABLE I Substrate temp., F.

Coating thickness Prior to End of (mils) deposition deposition Adhesion80 None. 150 300 Do. 250 450 Do. 350 530 Do. 450 630 None (40% re-evaporated). 550 670 None (50% re-evaporated).

At least one prior art reference had suggested that the adherence ofvacuum deposited coatings could be improved by pre-heating of thesubstrate. It may be seen, however, that at least with respect to Zn,such preheating is ineffective in improving coating adherence.

In the examples reported in Table II, specimens were post-heated atvarying heating rates. All speciments were quenched with water to 100 F.in less than two seconds. The total elapsed time from beginning ofheating to the end of the quench is reported.

ess, since such pressures may be obtained with mechanical pumps.However, to insure the adherence and appearance of the coating, it ispreferred to operate at chamber pressures below about 1 10 mm. Hg.

While a number of prior art processes have been mainly concerned withthe formation of a critical, intermediate iron-zinc alloy layer, theinstant method is apparently dependent on a totally different mechanism.In a particular experiment, two samples were identically vacuum-coatedwith Zn layers about 5 mils in thickness. Both specimens werepost-heated to 800 F. and quenched in water. However, therelapsed timefor one sample was 12 seconds and for the other 27 seconds. Thethickness (about 0.5 mil) of the intermediate iron-zinc alloy layerwhich formed, was exactly the same for both samples. However, as shownabove, the coating of the 12-second sample was adherent while that ofthe 27-second sample exhibited poor adherence. It therefore appears thatwhen longer elapsed times are employed, the loss in adherence is due tothe formation of voids at the interface as a result of the coalescenceof vacancies within the coating. This conclusion is borne out by thefact that even normally adherent, hot-dipped or electrodeposited Zncoatings lost adhesion when heated at 720 F. for an elapsed time inexcess of two minutes. The critical time-temperature relation forvacuum-deposited Zn coatings is much more limited however, since suchcoatings have more defects (vacancies, faults, etc.) than do hot-dippedor electroplated coatings. It therefore appears that the condensation ofvoids not only occurs during the post-heating of TABLE II CoatingMaximum Elapsed thickness substrate time (mils) temp., F. (seconds)Adhesion and remarks 0.7 500 6.0 Not adherent.

0.7 680 8.0 Poor.

0.7 750 7.2 Excellent.

1.3 800 9 Excellent.

2. 6 800 27 Poor, easily peeled from bend.

2.6 800 Ngnectlherent, fell off when 3.2 800 42 Very poor, very easilypeeled om ben 2.7 800 54 Nonadherent, fell off when bent.

The critical effect of achieving a temperature of at least about 750, aswell as that of elased time, may readily be seen.

The effect of elapsed time, as distinguished from heat-up rate is shownin the examples below.

TABLE III Time to Time to cool from Coating heat to 800 F. to thickness800 F. 100 F.

(mils) (sec.) (sec.) Adherence 0. 6 6 2 Excellent. 0. 5 7 1 120 Flakedofi. 0. 8 95 2 Do.

* Air cooled, rather than quenching in water.

Itis therefore seen, that even when rapid heating is employed (SampleNo. 2), an adherent coating 'Wll not result unless the specimen israpidly cooled so that the total, elapsed period is below the requisitetime.

Additional experiments showed that even when elapsed times of less than15 seconds were achieved, an adherent coating would not result unlesschamber pressure was maintained at below 4 l0- mm. Hg. Such a limitationwill not Seriously 8.5991 the economics of the instant procthe specimen,but also during the deposition of the Zn. Thus, a certain amount of postheating is essential in order to annneal out the so-formed voids, butthat excess post heating is counter productive.

The process of this invention may therefore be carried out in thefollowing manner. The ferrous article to be coated is cleaned, employingmethods conventionally employed by the art, such as in theelectroplating of various metals, e.g. Sn, Zn, Cr. The cleaned articleis passed to a vacuum chamber maintained at a pressure below about 1X10mm. Hg. While the article may be preheated, such a procedure is notrequired. For commercial applications (e.g. high speed plating) it isdesirable to deposit the Zn at very rapid rates. During such rapiddeposition, the temperature of the substrate will be substantiallyraised by the heat of condensation. Since the temperature of thesubstrate should not be permitted to reach the point at which thecoating would re-evaporate, it is preferred that the strip enter thechamber at about room temperature. However, as long as the substrate isin the vapor beam (directly over the crucible), temperatures of about600 F. (depending on chamber pressure) can be tolerated. When thedesired coating thickness is achieved, the article is removed from thechamber and heated in air to a temperature of about 750 to 900 F. andimmedi' ately quenched in water to a temperature below about 200 F.; theelapsed time from initiation of the heating to the consumption of thequench being no more than about 25 seconds. The upper limit of 900 F. isdetermined principally by considerations of appearance and corrosionprotection, rather than by adherence of the coating. Vacuum-deposited Zncoatings on wire specimens, heated in air to above 900 F. and quenchedto 200 F. in an elapsed time of seven seconds exhibited excellentadherence. However, the Zn coating exhibited excessive oxidation. Thatis excessive oxidation is meant to define the production of theresultant yellow oxide which was not only unattractive, but offered lessprotection as a sacrificial coating. If neutral or reducing atmospheresare employed (thus adding to the expense of the process) for thepost-heating step, then temperatures somewhat higher than 900 F. may beemployed. In any case, the temperature should be below 1000 F. toprevent the coating from burning otf.

Although the annealing of the voids effected by the instantpost-treatment is evidently a dilfusion process, it has surprisinglybeen found that the permissible elapsed time of the instant post-heatingprocedure, appears to be substantially the same for the entire range oftemperatures employed. Thus, a period of up to about 25 seconds ispermissible for maximum substrate temperatures of 900 F. as well as forsubstrate temperatures of 750 F. However, to insure that the criticaltime is not exceeded, it is preferable to achieve an elapsed time ofless than about 18 seconds.

We claim:

1. In the method for the coating of a cleaned ferrous substrate with acorrosion resistance layer of Zn, which comprises:

passing the substrate into a chamber maintained at a pressure no greaterthan about 4 10- mm. Hg and depositing Zn to a thickness of up to about7 mils on at least one surface of said substrate, by the condensation ofvapors issuing from a source of Zn maintained Within said chamber; theimprovement which comprises thereafter, passing said substrate out ofsaid chamber and heating said substrate to a temperature of from 750 F.to 1000 F. in an atmosphere and at a pressure sufiicient to prevent bothexcessive oxidation and the vaporization of the Zn coating; andthereafter cooling said susbtrate to a temperature below 200 F., saidheating and cooling being sufficiently rapid so that the elapsed timefrom the beginning of said heating to the conclusion of said cooling isnot greater than about 25 seconds. 2. The method of claim 1, whereinsaid substrate is at a temperature below 200 F. prior to thecondensation of said Zn vapors.

3. The method of claim 2, wherein said elapsed time is less than about18 seconds.

4. The method of claim 3, wherein said chamber pressure is below about 110- mm. Hg.

5. The method of claim 4, wherein said heating is conducted in air atabout atmospheric pressure, at a temperature of from 750 F. to about 900F.

References Cited UNITED STATES PATENTS 3,674,445 7/1972 Wlodek 117-1073,700,485 10/1972 Rubin 117--107 RALPH S. KENDALL, Primary Examiner I.W. MASSIE, Assistant Examiner US. Cl. X.R.

I W. MASSIE, Assistant Examiner UNITED STATES PATENT OFFICE CERTIFICATEOF CORRECTION 3,788,877 Dated January 29, 1978 Patent No.

Inventor(s) Lawrence E. Helwig et a1.

ppears in the above-identified patent It is certified that error ahereby corrected as shown below:

and that said Letters Patent are Column 5, line 3, "consumption" shouldread consummation Column 6, line LL, Claim 1, beginning of the line;

cancel "the improvement which comprises".

Signedand 'sealed this 18th day 51" June-"197E.

(SEAL) Attest: v I EDWARD M.FLETCHEB,JR. C. MARSHALL DANN AttestingOfficer Commissioner of Patents 7 FORM po-msouo-ss) ussoMwDc wand,

GOVERNMENT PRINTING OFFICE ll, O ''S6-3 l4,

